Prior Art
A continuing demand exists for the conversion of isobutane and light olefins into high octane motor fuels. The alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst, commonly referred to as HF alkylation, has provided a highly successful method for the production of high octane motor fuels. Despite a long history of safe operation, recent concerns over the possibility of a catastrophic release of HF acid from HF alkylation units has prompted the investigation of modifications or alternatives to the HF alkylation process for the production of motor fuels. One existing alternative is a similar alkylation process that uses sulfuric acid as the catalyst. While the use of sulfuric acid may decrease the degree of the hazard that some associate with the use of HF acid, sulfuric acid processes are still perceived as possibly presenting the same hazard and are not as economically advantageous as the HF alkylation process. Therefore, processing substitutes for the HF alkylation process are still sought.
Other methods of combining isobutane with light olefins to produce motor fuels are known and practiced; however, they do not produce the same quality gasoline products or are more expensive to install and operate. One such alternative is the dehydrogenation of butanes and the oligomerization of the resulting olefins to produce gasoline boiling range hydrocarbons. The oligomerization of light olefins into higher molecular weight motor fuels using a solid phosphoric acid is well known and its use predates the HF alkylation process. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Patents disclosing the combination of dehydrogenation of light paraffin streams with oligomerization of the resulting dehydrogenation effluent include U.S. Pat. Nos. 4,393,259; 5,049,360; 4,749,820; 4,304,948 and 2,526,966.
It also known to hydrotreat the olefinic hydrocarbon streams produced by oligomerization to saturate olefins. Patent GB 2186287 discloses dehydrogenation of and oligomerization of a C.sub.4 fraction to produce a jet aircraft fuel that is optionally hydrogenated. The hydrotreatment of jet fuels, diesel fuels and lubes produced by dehydrogenation and oligomerization of light paraffins is disclosed in U.S. Pat. No. 4,678,645. However, hydrotreating is not always beneficial for gasoline fractions produced by oligomerization and can lower octane ratings, but is known to be particularly beneficial when saturating isooctenes to isooctane gasoline.
An arrangement for producing alkylate quality feedstocks by the combination of dehydrogenation, oligomerization and saturation is disclosed in copending U.S. Ser. No. 08/573,089.
It is an object of this invention to provide an alternative to HF alkylation for the combination of isobutane with light olefins that advantageously produces a comparable motor fuel product in an integrated series of dehydrogenation, oligomerization and saturation steps in a more cost effective manner using a reduced amount of equipment.